The river Housatonic appears to have made its way through this range, for the same rock continues on the western side of the river parallel to it in the mountains of Kent, Sharon, and Salisbury. In Litchfield commences a range of porphyritic granite, or porphyritic gneiss, which alternates with the common gneiss, and in some instances rests upon it. This rock begins at Mount Prospect, between Litchfield and Warren, and runs through South Farms, Bethlem, and Watertown. The crystals of feldspar in it, are often very perfect.
The primitive granite, as a rock, is not found, though it lies scattered on the surface in great quantities, and large masses. The graphic granite in this region is often remarkably fine. Mica slate constitutes a considerable part of those rocks that rest on the gneiss, though never found in such elevated situations. The mica slate rocks are always inclined at a great angle with the horizon, and follow the direction of the other range. Litchfield village, Chesnut hill, and great part of Harwington, are entirely composed of this rock. The Bantum and the Waterbury rivers have their bottoms of it. Some of the brooks entering the Waterbury, have cut their passage through the mica slate, leaving walls of 40 or 50 feet on each side, traversed by veins of a very coarse-grained granite, and often much mixed with sulphuret of iron. The slate near Harwington meeting-house contains a great quantity of sulphuret of iron. Mica slate likewise lies on the sides of the gneiss range in Canaan and Salisbury, where it dips under the limestone. Sienite is scattered on the surface in large masses, especially where the porphyritic gneiss is found. Sometimes, however, the masses are so large as to form mountains. Mount Tom, between Litchfield and Washington, is of this nature, being entirely composed of sienite, resting on gneiss. Slaty sienite is frequently found, having a very large proportion of hornblende.
The minerals that are found in this region, are much more interesting than its geology. In describing them, I shall confine myself to the district east of the limestone range, intending at some future time to investigate and describe the limestone country.
Carbonate of lime, the granular limestone, is scattered over the whole of this region. It often is found in the cavities of decayed quartz rocks, and contains tremolite and augite.
Cyanite or Sappar, is found in great quantities, especially in Harwington and Litchfield. A crystalline mass of this was found a few years ago, weighing probably 15 cwt.; it lay on a mica slate ridge, and undoubtedly had been formerly imbedded in the slate. Beautiful white talc, and small crystals of sulphuret of iron, are disseminated in the mass. Specimens of this mass are in almost all the cabinets in America. Smaller masses have been found associated with feldspar. Small crystals of this mineral are very common in mica slate, with staurotide and garnet. Two of these crystals are often arranged at right angles with each other. In Cornwall it is found in small crystals in the gneiss containing graphite.
Staurotide is very common and very beautiful. It is found principally in mica slate, and exhibits often the cross. It most generally is crystallized in four-sided prisms.
Quartz, of course, is common. Cornwall particularly is distinguished for the smoky variety. Ferruginous quartz is found in rolled masses in the whole of this range.
Petro silex, in rolled masses with ferruginous quartz, containing veins of chalcedony and hornstone, and geodes of quartz crystals, are common in Litchfield and Goshen. Sometimes these masses in the interior assume the appearance of Burrstone.
Common opal has been found in Litchfield, though rarely. It was part of a mass of ferruginous quartz, with indelible dendritic impression. It is very hard, and its fracture is conchoidal.
Mica is very common. It is found green, white, and perfectly black. It generally occurs in blocks of granite.